Potassium channels are a large and diverse group of ion channels found in most eukaryotic cells. They control the electrical potential across the cell membrane and the excitability of nerve and muscle cells. Modulations of potassium channel activities represent one underlying mechanism for the regulation of neuronal signalling and synaptic plasticity. Despite the low abundance and extraordinary heterogeneity of potassium channels, two families of potassium channels have been cloned using genetic approaches and functional assays of channels expressed in Xenopus oocytes. The family of voltage-gated potassium channels has been analyzed molecularly over the past several years. This proposed study concerns structure- function analysis of the inwardly rectifying potassium channels which became amenable to molecular studies last year. How do these channels allow much larger potassium influx than efflux, no matter how much potassium is present in the extracellular medium? How can these channels allow potassium ions to go through their pore readily and yet have small ions such as sodium and magnesium to stall in the pore, thereby blocking potassium flux? How are certain potassium channels regulated by G protein in a membrane-delimited manner? These questions will be addressed by site-directed mutagenesis of inwardly rectifying potassium channels, and electrophysiological and biochemical studies using expression systems. A better understanding of how potassium channels allow selective potassium permeation, and how potassium channel activities are gated by G proteins and by the electrochemical driving force for potassium may facilitate analysis of potassium channel modulation, which is central to a wide range of physiological phenomena including parasympathetic regulation of heart rate, metabolic regulation of EEG arousal, and regulation of release of transmitters and hormones. Given that potassium channel blockers or openers have been used to treat diseases such as arrythmia, hypertension and diabetes, it conceivable that knowledge gained from structure-function analysis of potassium channels will also prove useful in the development of drugs and treatments for these diseases.